Compact quasi-elliptic lowpass filter using symmetric rectangular coupled capacitors

A compact microstrip lowpass filter using symmetric rectangular coupled capacitors is proposed. The introduction of an interdigital structure between the symmetric rectangular capacitors can reduce the filter size, owing to the increase in the loading capacitance, and also can provide the finite attenuation poles, thus quasi-elliptic function and stopband extension are fulfilled. Finally, a demonstration filter is designed, fabricated and tested to show the validity of the proposed lowpass filter structure.